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RBC parameters are used to differentiate beta thalassaemia traits (BTT) from other anaemias with different indices. The individual RBC parameters have shown a promise in differentiating BTT from other anaemias but we wanted a more precise way which suits our settings where the HPLC facilities are not available. We used the Fishers stepwise linear discriminant analysis.   The study was carried prospectively from November 2008 to March 2010 for randomly collected samples of MCV less than 80fl. HbA2 levels used as gold standard and other haemoglobinopathies were excluded. New index was calculated and the “Thal index” was then subjected to ROC curve analysis to identify the best cutoff to discriminate the BTT from other microcytics blood films. Software was developed to predict the BTT status (BTT prediction .exe).   The new index, referred to as the “Thal index”, was calculated using discriminant function analysis and is given as “Thal index” = (0.615 x MCV ) + (0.518 x MCH) + (0.446 x RDW). “Thal index” cutoff point of 59.0 has a sensitivity of 90% and a specificity of 85%.   This study showed a good sensitivity and a better specificity compared to other studies in world. Therefore this is a better answer to a set-up where the option of HPLC is not available to differentiate BTT from other microcytic blood films although this cannot replace HPLC. BTT prediction.exe is useful to get the instant prediction and the first ever computer program according to the available literature.